Honeycomb structures having traverse cross-sectional cellular densities of approximately ten to one hundred cells or more per square centimeter have several uses, including solid particulate filter bodies and stationary heat exchangers. Wall flow particulate filter applications require selected cells of the structure to be sealed or plugged by manifolding and the like at one or both of the respective ends thereof. The term “sealed” and other corresponding grammatical forms, i.e., sealant, sealing, etc., are used herein to refer to porous and non-porous methods of closing the open traverse cross-sectional areas of the cells.
The reference numeral 10 (FIG. 1) generally designates a solid particulate filter body that is generally well known and that may be fabricated utilizing a honeycomb structure 10 formed by a matrix of intersecting, thin, porous walls 14 surrounded by an outer wall 15, which in the illustrated example is provided a circular cross-sectional configuration. The walls 14 extend across and between a first end face 18 and an opposing second channels 22 which also extend between and are open at the end faces 18, 20 of the end face 20, and form a large number of adjoining hollow passages or filter body 10. The outer wall 15 (or skin) defines an outer edge 16 for both the first end face 18 and the second end face 20. To form the filter 10 (FIGS. 2 and 3), one end of each of the cell channels 22 is sealed, a first subset 24 of the cell channels 22 being sealed at the first end face 18, and a second subset 26 of the cell channels 22 being sealed at the second end face 20 of the filter 10. Either of the end faces 18, 20 may be used as the inlet face for the resulting filter 10 when all of the channels 22 are of the same size.
In operation, contaminated fluid (for example including particulate matter, such as exhaust soot) is brought under pressure to an inlet face and enters the filter 10 via those cell channels which have an open end at the inlet face. Because these cell channels are sealed at the opposite end face, i.e., the outlet face of the body, the contaminated fluid is forced through the thin porous walls 14 and into adjoining cell channels which are sealed at the inlet face and open at the outlet face. The solid particulate contaminant in the fluid, which is too large to pass through the porous openings in the walls, is left behind and a cleansed fluid exits the filter 10 through the outlet cells.
For the mass production of such filters and heat exchangers, it is highly desirable to be able to seal selected cell ends as rapidly and as inexpensively as possible. A known method of plugging includes the use of a mask having a number of openings extending therethrough for selectively manifolding honeycomb structures in the fabrication of solid particulate filter bodies (such as shown in U.S. Pat. Nos. 4,411,856; 4,427,728; 4,557,682; and 4,557,773). Heretofore, these masks have typically been used in conjunction with a foam-type cement that is formed into a paste by mixing ceramic raw material with an aqueous binder, such as methylcellulose, plasticizer and water (see U.S. Pat. No. 4,455,180, for example). When using this foam-type cement, both ends of the honeycomb structure are covered with flexible or rigid plates having holes through which the cement is pushed into the ends of the cells. In one particular application, the cement is pressed through an associated mask by a servo-driven piston based plugging machine (See FIG. 1 of U.S. Pat. No. 4,557,682), wherein the machine displaces a charge of cement to create the required plugs within the cell channels.
There are numerous disadvantages to this particular process, including the high variability in overall plug length, misalignment of the mask with respect to the filter body, thereby causing unplugged cells, and the creation of unplugged cells due to dry cement that “flakes-off” into fresh cement prior to application. This process generally uses masks pre-formed of silicon that are reused for plugging multiple honeycomb structures. Reusable masks can be less effective at plugging the cells because they cannot adjust to variations between the honeycomb structures. The equipment used in these conventional processes is also incompatible with laser-cut polymer masks as these laser-cut masks stay with the cemented body when the associated plugging machines open for part release (See U.S. Pat. No. 4,557,773). Another problem with silicon masks (as shown in U.S. Pat. No. 4,427,728) is that they are sized so as to align with the outer edges of the filters, thereby allowing the cement to flow along and adhere to an outer edge of the filter.
Masks have also been formed for manifolding cells that are regularly interspaced among substantially mutual parallel rows and substantially mutually parallel columns at an open face of a honeycomb structure by applying strips of adhesive backed flexible webbing impermeable to the sealed material, such as masking tape, over selected rows and columns of cells. Alternatively, these cells are created by providing a matrix of spaced, overlaid strips of resilient, impermeable and reusable material such as metal foil, which are then joined together and fitted, with or without an underlying gasket, over the open face of the structure with the openings through the matrix and gasket position opposite the cell channels to be charged. By providing a honeycomb structure with cells arranged in mutually parallel rows and mutually parallel columns and covering alternative rows and alternative columns of cells with strips of suitable flexible materials such as masking tape or the joined thin metal strips, the open ends of half of the subset of cells arranged in a checkered pattern across the open face are exposed. After filling the ends of the strips, the strips are removed and strips are applied covering the remaining alternative rows and remaining alternative columns, thereby exposing the open ends of the remaining half of the subset of cells of the checkered pattern of the end face for filling.
Both of these embodiments provide greater flexibility in dealing with the surface height variations and provide better masking of the cell ends not to be charged, including those which may be damaged, than does the rigid plate embodiment. However, both embodiments must typically be applied twice to each end face. This is a significant limitation with respect to the tape strips which must be individually applied across each end face, a time-consuming task. The reusable matrix and gasket of the second embodiment may be more quickly applied and removed, but like the rigid plate embodiments, is less easily adapted to distortions in the cell locations of the end faces. Moreover, increasing cellular densities render such an approach unworkable.
Additionally, variations in the surface of the plugging material on the plugging apparatus can be transferred into the substrate causing variations in the length of intrusion of the plugs. Therefore, it is desirable to have an initial surface that is substantially flat and pushed in with uniform force to allow all resulting plugs to be similar in length. The length of intrusion should be deep enough to secure a good blocking of the sealed channels, but is desirably limited because an increase in plug length results in the loss of porous wall surface area.
A method for manifolding or plugging extruded honeycomb structures, such as ceramic particulate traps for diesel engines, is desired that uses the existing cement composition and rheology, is compatible for use with lasercut polymer masks, reduces plug length variability, minimizes missing plugs, and eliminates overflow of the plugging material from the cell channels to which the plugging material is applied. The method should also be highly repeatable and accurate, easily applied, and reduce unintended deformations of the honeycomb structure.